1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use in superheterodyne system and a communications apparatus including such a filter.
2. Description of the Related Art
In general, with a mobile communications apparatus such as an automobile telephone and a cellular telephone, a superheterodyne system is used to obtain a high sensitivity and a high stability. This system uses a phenomenon in which, as soon as two signal waves f1 and f2 are mixed together so that it is possible to obtain signal waves (f1xc2x1f2) representing a sum and a difference. This system also involves converting a received signal wave into a low frequency signal. A typical communications apparatus using the system is shown in FIG. 16.
As shown in FIG. 16, in the communications apparatus 100, a high frequency module 120 is connected with an antenna 110, and a signal processing circuit 130 is connected at a rear section of the high frequency module 120.
The high frequency module 120 includes band pass filters 121 and 122, two amplifiers 123 and 124, a mixer 125 and a local oscillator 126.
The first band pass filter 121 may be arranged to select a pass band such that the filter will allow the passing of signals received from the antenna 110, but will attenuate other signals not received from the antenna 110.
The amplifier 123 is provided to amplify a received signal which has passed through the first band pass filter 121.
With the local oscillator 126, the frequency of its local oscillating signal is set such that a sum of the local oscillating signal and the received signal or a difference between these two signals will become a desired intermediate frequency signal.
The mixer 125 is used to mix the local oscillating signal fed from the local oscillator 126 with the received signal, so as to convert these signals into intermediate frequency signals.
With the second band pass filter 122, the pass band is selected such that it attenuates image frequency signals other than the desired intermediate frequency signal generated when the local oscillating signal and the received signal are mixed together in the mixer 125, thereby allowing the passing of the desired intermediate frequency signal.
The second amplifier 124 is used to amplify the intermediate frequency signal which has passed through the second band pass filter 122, thereby transmitting the signal to the signal processing circuit 130 connected at the rear section of the high frequency module 120.
The signal processing circuit 130 is used to process the intermediate frequency signal transmitted hereto, so as to convert the signal into, for example, a sound or the like.
In such a communications apparatus 100, the frequency of the image frequency signal (hereinafter, it will be referred to as xe2x80x9cthe image frequencyxe2x80x9d) will be determined depending upon the frequency of the local oscillating signal of the local oscillator 126 and the center frequency of the second band pass filter 122. However, such an image frequency will usually be generated at a position that is separated by several hundred MHz from the pass band of the second band pass filter 122.
Conventionally, as a second band pass filter 122, as shown in Japanese Unexamined Patent Publication No. 5-183380, there has been mainly used a ladder-type surface acoustic wave filter connected in a ladder configuration with a plurality of surface acoustic wave resonators.
The ladder-type surface acoustic wave filter includes an area having a high attenuating amount, which area is originally located in the vicinity of its pass band. According to the Japanese Unexamined Patent Publication No. 5-183380, if an inductance is added to the resonators arranged in parallel, it is possible to increase the pass band width of the filter, thereby rendering the high attenuating area to shift towards a low frequency band, thus improving the attenuation characteristic of the low frequency band located in the vicinity of the pass band. However, with the method, since the width of the pass band will at the same time be increased, the filter is not suitable for attenuating image frequency signals generated at positions which are separated by several hundred MHz from the pass band of the filter.
Further, as described in Japanese Unexamined Patent Publication No. 9-261002, an inductance component caused by connecting means such as bonding wires and a capacitance component caused by a connecting portion of a bonding pad with respect to a reference potential, may be both interposed and inserted in parallel with each other, so as to provide a high attenuating area at a desired point on a low frequency band side of the pass band, while at the same time continuously preventing an increase of the width of the pass band.
However, with the method of Japanese Unexamined Patent Publication No. 9-261002, although it was possible to attenuate the image frequency occurring on the low frequency band side of the pass band, it was still difficult to attenuate the image frequency occurring on the high frequency band side of the pass band.
Further, in order to obtain the capacitance component, it is necessary to increase the connecting portions of the bonding pad and floating electrodes. For this reason, a surface acoustic wave element must have a large size, hence making it difficult to produce a surface acoustic wave filter which is compact.
Moreover, if it was desired to obtain a sufficient capacitance component and to prevent any increase in the size of the surface acoustic wave element, it would be necessary that the floating electrode and the connecting portions of the bonding pad be disposed without any slots formed therebetween, resulting in a reduced freedom of design and arranging various elements and connections thereof.
To overcome the problems described , preferred embodiments of the present invention provide a surface acoustic wave filter capable of attenuating an image frequency, regardless of whether it is generated on a high frequency band side or a low frequency band side of a pass band, without having to increase the size of the surface acoustic wave element.
The surface acoustic wave filter according one preferred embodiment of the present invention includes at least one surface acoustic wave element including a piezoelectric substrate, at least one IDT provided on the piezoelectric substrate, input/output terminals provided on the piezoelectric substrate and reference potential terminals provided on the piezoelectric substrate and both of which are connected with the at least one IDT, a package enclosing the surface acoustic wave element, the package including electrode lands respectively connected with the input/output terminals and the reference potential terminals of the surface acoustic wave element, external terminals connecting the electrode lands on the outer circuit thereof; the surface acoustic wave filter being adapted for use in a superheterodyne system, wherein the resonance frequency of the resonator having the capacities of IDTs of the surface acoustic wave element and having inductances from the reference potential terminals of the surface acoustic wave element to the external terminals of the package, is positioned in the vicinity of an image frequency occurring at the time of performing frequency conversion in the superheterodyne system.
According to the described structure of this preferred embodiment, since inductances are added in series with the capacities of the IDTs, it is possible to attenuate a desired image frequency.
Further, the resonance frequency of the trap resonator may be attenuated either on a high frequency band side or a low frequency band side of the pass band by setting the capacities of the IDTs and the values of the inductances in series therewith, thereby making it possible to cope with a high frequency of the intermediate frequency signal.
Moreover, since only the resonance frequency of the trap resonator is changed, it is possible to attenuate the image frequency without affecting an insertion loss in the pass band.
Further, since the capacity bonding wires of the IDTs and the inductance of the package and the like are used to define a trap resonator with the use of the present structures thereof, it is easy to make the entire surface acoustic wave filter compact. Further, the frequency difference fd1 between the center frequency of the at least one of the surface acoustic wave elements and an image frequency occurring at a time of performing frequency conversion in a corresponding superheterodyne system, is made to be approximately equal to the frequency difference fd2 between the center frequency of another of the at least one of the surface acoustic wave elements and the image frequency occurring at a time of performing frequency conversion in a corresponding superheterodyne system. Therefore, it is possible to use common circuits for signal processing from the intermediate frequency signals onward, with respect to a plurality of filters which include a plurality of surface acoustic wave elements. Thus, it is possible to reduce the number of the parts and elements required to construct the device.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.